Oscillators are used in radio communication devices to provide a variety of functions such as the generation of a local oscillator signal. Resonators are used in oscillators as the means of generating a free running signal. Resonators are available in a variety of topologies. One style of resonators utilizes a micro-strip followed by an inductor. This particular resonator style is suitable for applications where tunability is desired. In general, variation in component tolerance and parasitic as well as variation over temperature requires that frequency of oscillators be adjusted in a post assembly process. From a performance stand point using a fixed tune method, capacitive or inductive is the most desirable. The trim range of an inductor micro-strip component is proportional to the effective length/width variation of the component caused by the adjustment process. This tuning range is constrained by the physical size of the component, the parasitic capacitance, and the degradation of Q factor caused by narrowing the line when tuning.
Current methods of inductive tuning use a single cut into a metalized area. The tuning area required for this approach would make it necessary to increase the size of the tuning element. This increased size has two problems. First, the thermal stress on the alumina to FR4 solder connections would cause solder cracking and a consequent reduction in the mean time before failure (MTBF) for the product. Second, the decrease in the characteristic impedance of the line would reduce the varactor tuning range for a given circuit.
These problems combined with others render this tuning style undesirable. An improved method of tuning resonators is therefore desired.